1. Technical Field of the Invention
The present invention relates to an organic electroluminescent (EL) display device utilizing an electroluminescence of an organic compound to display images.
2. Description of the Related Art
An organic EL display device is a device having a plurality of display elements arranged in a matrix pattern for displaying an image by selectively actuating the individual display elements for light emission. As shown in FIG. 7 illustrating an example, a display element 9 includes a transparent electrode segment 91, an organic laminate 92, and a reflective electrode segment 93 successively formed on a glass substrate 90 in the mentioned order. The transparent electrode segment 91, serving as an anode, is made of e.g. ITO. The organic laminate 92 includes a light emitting layer 92A which emits light upon application of a voltage. In the illustrated example, the organic laminate 92 further includes a hole transport layer 92B and an electron transport layer 92C in addition to the light emitting layer 92A. The reflective electrode segment 93, serving as a cathode, is made of e.g. a highly reflective material such as aluminum or copper.
The display element 9 generates light in the light emitting layer 92A by applying a voltage across the organic laminate 92 utilizing the transparent electrode segment 91 and the reflective electrode segment 93. The light generated in the light emitting layer 92A propagates in directions Z1, Z2 of FIG. 7. The light component propagating in the direction Z1 passes through the hole transport layer 92B, the transparent electrode segment 91 and the glass substrate 90 to be emitted out of the display element 9. The light component propagating in the direction Z2 passes through the electron transport layer 92C, reflects at the reflective electrode segment 93, and then goes back through the organic laminate 92, the transparent electrode segment 91 and the glass substrate 90 for exiting out of the display element 9.
Thus, the display element 9 can emit only the light component passing through the glass substrate 90 after passing through at least the hole transport layer 92B and the transparent electrode segment 91. Consequently, only a single surface can be utilized as a displaying face in the organic EL display device incorporating the above-described display element 9.
In addition, the light generated in the light emitting layer 92A has to pass through many elements before exiting out of the display element 9, so that some portion of light cannot be utilized due to absorption before emission.
In order to achieve a double-face display in an organic EL display device, it is conceivable to provide both of the anode segment and the cathode segment of the display element as transparent electrodes, for example. In this case, when both the anode segment and the cathode segment are made of ITO, a high driving voltage is needed due to a relatively large resistance of ITO. Further, since ITO tends to transmit a long-wavelength component of visible light more easily than a short-wavelength light component, light emitted from the display element becomes reddish to adversely affect hue of colors. A solution for such a problem requires the use of an optical film, causing a disadvantage. In particular, this problem becomes more remarkable with respect to double-face display which requires two optical films.